Abnormal p53 expression in benign breast tissue is associated with the subsequent development of breast cancer and may represent a very early event in breast carcinogenesis. While the importance of p53 as a tumor suppressor is well established, little is known about the fate of normal human mammary epithelial cells (HMECs) that acutely loose p53 function in the context of extracellular matrix-derived growth and differentiation signals. Loss of epithelial polarity and dysregulated growth (mammary hyperplasia) are the first morphologic abnormalities observed during the progression from normal mammary architecture to invasive breast cancer. Interactions between mammary epithelial cells and extracellular matrix (ECM) play a critical role in maintaining normal tissue homeostasis and are likely disrupted in mammary hyperplasia. We developed an in vitro system to investigate how ECM-signal transduction might act in HMECs that have acutely loss p53 function. We observe in Preliminary Data, that ECM-growth arrest and polarity signals may act to initiate apoptosis in HMECs with HPV-16 E6-suppressed p53 expression. Resistance to ECM-mediated growth arrest and epithelial polarity is associated with resistance to apoptosis. We hypothesize that ECM-derived signals may play an important role in preventing the clonal expansion of abnormal mammary epithelial cells by promoting apotosis of cells that acquire p53 mutations and therefore, resistance to ECM-signaling might promote breast carcinogenesis by inhibiting apoptosis. Our model would predict that 1) HMECs acquiring a p53 mutation in the context of a normal polarized epithelium will be eliminated by apoptosis but 2) when HMECs acquire a p53 mutation in the absence of normal ECM growth regulation and polarity signals (mammary hyperplasia) aberrant cells will not be eliminated by apoptosis. Specific Aim I, will test the requirement for p53 loss in ECM-mediated apoptosis: 1) p53 function in HMECs will be suppressed by non-viral approaches and 2) suppression of the p53-regulatory proteins CBP/p300. Specific Aim II, will test whether loss of epithelial polarity and resistance to ECM-mediated growth arrest results in resistance to apoptosis utilizing two in vitro human models of mammary hyperplasia. Suppression of ECM-mediated growth arrest and epithelial polarity will be accomplished by retroviral-mediated expression of dominant-negative retinoid receptor and by treatment of three dimensional ECM cultures with alpha6- and beta4-integrin blocking antibodies.